This invention relates to the conversion of energy from naturally occurring sources of mechanical energy, such as the mechanical energy present in ocean surface waves, to electrical energy. This invention also relates to the efficient production of the electrical energy and the transfer of that energy.
Various wave energy converter (WEC) systems are known. For example, reference is made to U.S. patent application Ser. No. 09/379,421 filed Aug. 21, 1999, titled xe2x80x9cWave Energy Converter Utilizing Pressure Differencesxe2x80x9d, and to U.S. patent application Ser. No. 09/922,877 filed Aug. 6, 2001, titled xe2x80x9cApparatus And Method For Optimizing The Power Transfer Produced By A Wave Energy Converter (WEC)xe2x80x9d, both assigned to the assignee of the present application and the teachings of which are incorporated herein by reference and/or explicitly.
Numerous problems exist in the design of a mechanical system for harnessing the energy contained in ocean waves. Particularly, a problem exists in harnessing the energy efficiently. Furthermore, the problem includes determining criteria for certain physical characteristics of the structure and system to ensure that the energy contained in ocean waves is harnessed most efficiently and what should be done to optimize the power generated and the power transferred.
Applicants"" invention resides, in part, in the recognition that the efficiency of a wave energy converter (WEC) which includes a tubular structure and a piston within the tubular structure where the relative motion between the piston and the tubular structure is used to generate electric power may be optimized by limiting the length (L) of the tubular structure to certain values. In accordance with the prior art, the length of the tubular structure was made as long as possible for a given water depth, Dw. However, Applicants discovered that this should not be the case. Applicants recognized that: (a) the efficiency of the power transfer from the input wave to the output of the generator increases as the length xe2x80x9cLxe2x80x9d of the tubular structure increases from a minimal value until L reaches an optimal value; and (b) the efficiency decreases as L is increased beyond the optimal value due to the increased mass of the water that the tubular structure and the piston have to move.
Where the WEC is intended for use in a body of water having a water depth (Dw), with peak-to-peak wave heights of H, and where the waves have a wavelength of xcex, the length L of the tubular structure may be expressed as a function of Dw and xcex.
In the case of a cylinder, the optimum value of L of the tubular section can be found by determining the solution of L as set forth below.       L    OPT    =            1      -              cosh        ⁡                  (                      2            ⁢                          xe2x80x83                        ⁢                                                            π                  ⁢                  L                                OPT                            /              λ                                )                    +                        tanh          ⁡                      (                          2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                                                D                  w                                /                λ                                      )                          ⁢                  sinh          ⁡                      (                          2              ⁢                              xe2x80x83                            ⁢                                                                    π                    ⁢                    L                                    OPT                                /                λ                                      )                                                                                                                    (                                      4                    ⁢                                          π                      /                      λ                                                        )                                ⁢                                  sinh                  ⁡                                      (                                          2                      ⁢                                              xe2x80x83                                            ⁢                                                                                                    π                            ⁢                            L                                                    OPT                                                /                        λ                                                              )                                                              +                                                                                          (                                  4                  ⁢                                      π                    /                    λ                                                  )                            ⁢                              tanh                ⁡                                  (                                      2                    ⁢                                          xe2x80x83                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                                                                  D                        w                                            /                      λ                                                        )                                            ⁢                              cosh                ⁡                                  (                                      2                    ⁢                                          xe2x80x83                                        ⁢                                                                                            π                          ⁢                          L                                                OPT                                            /                      λ                                                        )                                                                        ⁢              xe2x80x83            
Alternatively, the optimum value of L of the tubular section may be obtained from calculations for the power output of the system as a function of the length of the tubular section for a given depth Dw of the body of water. The calculations indicate that as the length of the tubular section increases from zero to a value equal to the depth Dw of the body of water, the power output increase to an optimal value and then decreases. The results indicate that there is a shallow peak, whereby a power output of approximately 90% of the peak value may be obtained for values of L ranging between 0.3Dw and 0.6Dw. In addition, for xe2x80x9cdeep waterxe2x80x9d condition (where Dw/xcex greater than ⅓), L may be a function of xcex. For example, L may be equal to 0.2xcex.
These results enable the designer to select the most attractive length of the tubular section with due consideration for the power output and the cost of the system; since typically the cost of the WEC increases as its length increases.